The perception of pain is not always a true reflection of peripheral- tissue damage. There are many syndromes of severe, chronic pain that are disproportionate to the injury or condition that initiated the pain. Likewise, there are situations in which the central nervous system is capable of inhibiting the transmission of pain related information from the periphery. Acutely stressful circumstances, characterized by sympathetic nervous system (fight-or-flight) activation, are known to produce effective analgesic states in laboratory animals and humans, although relatively little is known about the common circumstances that activate endogenous pain inhibitory mechanisms in humans. This proposal will focus on the pain inhibitory effects associated with athletic competition that has been documented recently in college athletes as a model for activating the stress induced analgesia substrate in humans. The implication of athletic competition-induced analgesia is that pain loses its protective function when its inhibition allows for continued play, potentially aggravating and increasing the severity of injury and possibly contributing to chronic pain conditions and dependence on pain relieving drugs. The propensity to continue competing despite injury takes on added significance in the context of sport as a for-profit industry. The specific aims of the proposed research are to determine the analgesic (and/or hyperalgesic) effects of various types of athletic competition in male and female athletes using pain measurement tools that will allow for analysis of the psychophysical function, thereby providing an accurate measure of the degree of analgesia observed; understand the contributions of the different stressful components of athletic competition (that related to competition only, and that related to exercise) to the pain inhibitory effects of such competition in male and female subjects: determine the lone-term and short-term time course of the analgesic effects of stressful whether individual differences in pain threshold and/or tolerance contribute to athletic statu.